In 1962, a young graduate student at McGill University named Suzanne Corkin met a 35-year-old man named Henry Molaison. Molaison and his mother had come to Montreal from their home in Connecticut so that Ms. Corkin and her colleagues could run a week-long series of psychological tests on him. Molaison was a sweet, cooperative subject. Ms. Corkin and Molaison talked a lot that week, mostly about Molaison's childhood. Beyond his early years, though, Molaison's recollections faded away.

The reason was both simple and profound. As a child, Molaison had suffered from severe epilepsy. In 1953, a surgeon took the drastic step of removing the part of his brain that was believed to be the cause. While the surgery helped reduce the seizures, it also had a devastating side effect: For the rest of his life, Molaison could hold most new information in his mind for only 30 seconds or so.

After that first meeting in 1962, Ms. Corkin continued to meet with Molaison—first at McGill and then at MIT, where she is a professor—until his death in 2008. In those 46 years, the two of them made scientific history, finding answers to some fundamental questions about how the brain works.

As a tribute to Molaison, Ms. Corkin has written "Permanent Present Tense," a remarkable blend of biography, memoir and scientific history. To convey just how important Molaison was to neuroscience, Ms. Corkin delves into early theories about the mind and memory. Nineteenth-century scholars such as William James made educated guesses about how memory worked, based mostly on their own subjective experiences. In the 20th century those speculations became more sophisticated, drawing on the emerging understanding of how neurons function. But these ideas had little grounding in the anatomy of the brain and thus lacked an accounting of what happened where.

Even in the mid-1900s, it was hard to get a glimpse at how a living brain worked. Wilder Penfield, a Canadian brain surgeon, used his surgeries as opportunities to map his patients' brains. By applying an electrode to certain spots, he found, he could trigger twitches and tingles in different parts of patients' bodies. The only other way to learn about the parts of the brain was to study people who were missing them. Strokes, tumors and surgery could all subtract chunks of neurons. Many of those affected held on to most of their faculties. But they also lost a part of themselves. Some could no longer speak. Others no longer recognized faces. Still others lost fear.

Molaison lost part of his memory. His surgeon had removed much of a structure in the brain called the hippocampus. At the time no one was sure what happened in it. As Molaison recovered in the hospital, his mother realized something was very wrong. Her 27-year-old son could not remember the nurses. Nor could he remember momentous events in his life, like the death of his uncle three years earlier.

The McGill psychologist Brenda Milner led the first studies on Molaison, and in the 1960s Ms. Corkin started working with him as well. From the start, their studies were a revelation. Molaison could only keep track of a conversation for short chunks. And yet he could learn new things. In 1966, he was able to draw a map of the house that he and his family moved to in 1958—five years after his operation. Ms. Corkin called the current owners of the house and discovered that the map was spot on. Most remarkably of all, Molaison developed so strong a sense of familiarity with Ms. Corkin that he came to think they had gone to high school together.

It would be wrong, in other words, to say that Molaison lost his memory. That's because memory is not a single thing but an intricate system of different processes in the brain, only some of which occur in the hippocampus. The experiments that Ms. Corkin and her colleagues conducted allowed them to dissect memory into its parts. Working memory—the information we take in and use right away, such as the digits of a new phone number—depends on structures in the prefrontal cortex. The hippocampus, Ms. Corkin and others showed, is crucial for creating and retrieving long-term memories—specifically, declarative memories that we can consciously access. Learning to perform a physical task or becoming familiar with a place, by contrast, depends on other regions of the brain. Ms. Corkin's research helped establish these hypotheses, and later research—with healthy subjects and brain-imaging technology—confirmed many of them.

At first, Molaison's story appeared only in scientific journals. Other scientists knew of him as "H.M." But over the years, his fame grew. He appeared in documentaries, tranquilly unaware for the most part of how he was different, and he was the subject of a 1995 book. After his death in 2008, Molaison became even more famous, with his obituaries revealing his real name at last. Reporters and television producers trailed Ms. Corkin to the Boston airport as she lugged a cooler containing Molaison's freshly removed brain. After being flown to Los Angeles, it was sliced and photographed at the University of California, San Diego, to create an online archive of its anatomy. The event was live-streamed.

"Permanent Present Tense" stands as the definitive story of Molaison. Ms. Corkin's narrative is rich with tales of his life, from his happy childhood to his painful decline in later years. While Molaison struck Ms. Corkin during his visits as sweet-tempered, she doesn't shy away from his occasional outbursts of violence and anger. But she uses those stories to speculate about how they might have been triggered by his amnesia.

Molaison even comes off as sly at times. In 1985, a researcher wanted to test his perception of time. She told him that she was going to leave the room and return and then ask him how long she had been out. Molaison noticed that a clock on the wall read 2:05. While the researcher was gone, he said "2:05" to himself, over and over, until she returned at 2:17. When she asked him how much time had passed, he declared, to her astonishment, "twelve minutes—got you there!" Even that prank revealed to Ms. Corkin and her colleagues something about the brain's ability to improvise.

"Permanent Present Tense" is a great book, but it could have been a masterpiece. It falls short, ironically enough, in Ms. Corkin's own experiences. Writing about Molaison's life, she can be enormously empathetic; her descriptions of neuroscience are often vivid. Her metaphors are delicious—she likens the signals that preserve a memory in the brain to "a conversation kept alive by a group of people standing in a circle." When she recounts seeing brain surgeries for the first time as a graduate student, she describes how surgeons placed tiny letters on patients' brains to mark places vital for language and other functions.

But too often Ms. Corkin slips into a clinical, unemotional voice. This is a tone that as a science writer I hear all the time, and I understand why scientists strive for it. They want to be objective and unassuming. But Molaison's story is drenched in emotion—happiness, grief, fear, fascination. And Ms. Corkin's own life became intertwined with Molaison's. She spent much of her life observing his life at close quarters, yet she doesn't delve much into how that experience affected her. "Permanent Present Tense" left me hungry with curiosity about both its leading characters.